Looking Up: Telling time under the stars

Tuesday

Dec 23, 2008 at 12:01 AMDec 23, 2008 at 11:13 PM

What time is it? If you left your watch at home, and the stars are out, just look up. You can learn at least a general idea of the hour, knowing the time of year. Your clock is the northern sky and the “hands” of the clock face are imaginary lines you can trace from the clock center, which lies right next to the North Star.

Peter Becker

What time is it? If you left your watch at home and the stars are out, just look up. You can learn at least a general idea of the hour, knowing the time of year. Your clock is the northern sky, and the “hands” of the clock face are imaginary lines you can trace from the clock center, which lies right next to the North Star.

On opposite sides of the North Star are the familiar seven stars of the Big Dipper, and the M-shaped constellation Cassiopeia. At 7 p.m. in late December, Cassiopeia is straight on top, almost overhead. The Big Dipper’s handle will be scraping the northern horizon.

This assumes that you live in mid-northern latitudes. If you live far to the north, the Big Dipper’s low point is much higher and doesn’t come near the flat horizon. The “sky clock,” however, is useful anywhere on Earth.

As Earth spins, the stars appear to move from east to west, circling a point next to the North Star. Since Earth rotates in 24 hours, the whole sky seems to spin once around in 24 hours. The sky clock then is a 24-hour clock.

Frequently the two front stars of the Big Dipper’s bowl are used to mark the clock’s pointer, since an imaginary line from these two stars happens to point to the North Star. You could also use an imaginary line through the center star of the “M” of Cassiopeia, connecting with the North Star.

Our sky clock has but one hand, and hopefully unlike the watch on your wrist, this clock runs counter-clockwise. Also because it is a 24-hour clock, it takes six hours for the stars to move one-quarter of the way around. A 12-hour clock takes six hours to go halfway.

Starting out by knowing the time on your wristwatch, note where the sky clock “pointer” is, whether you use Cassiopeia’s middle star or the Big Dipper’s front stars. Through the night, you can then get a close estimate of the time elapsed. A circle is divided into 360 degrees. Each hour that passes on the sky clock, the “pointer” moves 1/24th of the way, or 15 degrees.

As you look at the sky clock from week to week, you will see that the position of your reference stars changes as seen at the same hour each night. This is due to Earth’s revolution around the sun. We are constantly facing a slightly different perspective of the night sky as Earth moves around in its orbit. Since Earth takes 12 months to go around, and there are 12 hours on your wristwatch, the reading of the giant sky clock will change one hour per month to a person watching at the same time each night.

Here’s how to tell the time more exactly. Although you can get an estimate of the time this way, note that to be accurate a few adjustments are needed. The sky clock runs fast, by approximately 24 hours every year. This is because while there are 360 degrees measured in a circle, Earth takes roughly 365 days to go around the sun. This makes Earth move ahead by about one degree in its orbit every day, which results in the sky clock pointer moving fast counterclockwise by about one degree every day. That amounts to a half-hour a week.

The sky clock agrees exactly with real time once a year, on March 8 at midnight. At that time, the Big Dipper’s pointer stars are straight overhead. Six hours later, or 6 a.m., the Big Dipper will be a quarter of the way around. (If you use Cassiopeia, the “M” will now look like a “W” close to the northern horizon at midnight March 8.)

Using the March 8 date to calibrate your sky clock, you can find the time by subtracting two hours for each month from March 8. Subtract 30 minutes for each week, and another four minutes for each remaining day. Don’t forget to ADD an hour when daylight savings time is in effect. With cardboard and a protractor, one can build a crude sextant to closely measure the angle of the star positions around the North Star, and better tell what time it is.

To further complicate matters, Earth does not rotate exactly in 24 hours, leading to a year of about 365-1/4 days. This is why every four years we have Leap Year. Because Earth is gradually slowing down due to friction with the tides, officials at the International Earth Rotation and Reference Systems Service add a “leap second” every now and then. One is being added this year, on Dec. 31, at 6:59:59 p.m. EST (or just before midnight at the longitude of Greenwich, England, where we measure the start of every day).

Use your extra second wisely.

New moon is on Dec. 27, and first-quarter moon isn’t till next year (Jan. 4).

Keep looking up!

Peter W. Becker is managing editor at The Wayne Independent in Honesdale, Pa. He has been an amateur astronomer since the age of 12, in 1969. He may be reached at pbecker@wayneindependent.com.